Government plans to part-privatise Royal Mail have been roundly rejected by the Labour party membership by a margin of three to one, according to a poll conducted by YouGov.

The poll comes as Gordon Brown prepares to face tough questioning over the legislation from union leaders at a meeting of the party's national policy forum in Bristol today. The prime minister remains strongly committed to the reforms that he regards as necessary if Royal Mail is to gather the investment and management expertise to modernise.

He also believes the public, if not currently the party, will support the reforms. His aim is to whittle down the level of opposition on the backbenches by offering concessions.

The unions are due to hold a rally on the issue at the policy forum, but it is not formally on the agenda of the forum itself. The unions will be buoyed by the poll findings as a sign that their high-profile campaign has the support of the ordinary individual membership.

The poll, commissioned by the leftwing Compass group, was conducted by YouGov on Thursday among 911 party members. It found that 74% were satisfied with Royal Mail and only 14% dissatisfied. Two thirds said it should stay wholly publicly owned and only 24% supported selling a minority stake, as Lord Mandelson, the business secretary, proposes.

Only 30% thought Mandelson's plans were consistent with the party's election manifesto and 60% said it represented a breach of the manifesto pledge

Asked who should pay for Royal Mail's £6bn-plus pension deficit, 46% said it should be funded through higher taxes, extra borrowing or cut elsewhere. Only 26% said the deficit should be funded from private sector investment and 12% said by cutting pension payouts to staff.

Nearly 61% believe Royal Mail should not be split, but remain a single entity.

Neal Lawson, the chairman of Compass, said: "Labour MPs who want to honour the pledge in 2005 to keep the Royal Mail wholly public will be heartened by these poll results. They will know the party membership is behind them and not the government."

At least seven ministerial aides have put their names to a Commons motion opposing the controversial policy - with several indicating they might quit over the issue.

The prime minister's special envoy to Iraq, Ann Clwyd, has also signed, bringing the total number of Labour rebels to 136. She said yesterday that she would wait and see what concessions the government offered before deciding whether to step down.

Union leaders plan to tackle the prime minister after he has delivered a keynote speech to the conference, as well as raising the subject during a seminar later in the day.

Postal workers will stage a demonstration outside the meeting and will point out to those attending that Labour has a policy of keeping Royal Mail wholly publicly owned.

Mandelson published the bill paving the way for a partial sale of Royal Mail this week, insisting it would remain in public hands even though up to 30% could be sold to a foreign company.

Paul Kenny, general secretary of the GMB, who will attend the policy forum, said: "All parts of the Labour party, from Tony Blair, the parliamentary Labour party, constituents and unions, agreed in 2004 that the Royal Mail would continue to belong to the people, and this pledge was included in the election manifesto put before the public in 2005.

"Lord Mandelson has no democratic mandate whatsoever to change this undertaking."